Commit db4df481 authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab Committed by Jonathan Corbet

futex-requeue-pi.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:

- promote level for the document title;
- mark literal blocks.
Signed-off-by: 's avatarMauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: 's avatarJonathan Corbet <corbet@lwn.net>
parent af7175bc
================
Futex Requeue PI
----------------
================
Requeueing of tasks from a non-PI futex to a PI futex requires
special handling in order to ensure the underlying rt_mutex is never
......@@ -20,28 +21,28 @@ implementation would wake the highest-priority waiter, and leave the
rest to the natural wakeup inherent in unlocking the mutex
associated with the condvar.
Consider the simplified glibc calls:
/* caller must lock mutex */
pthread_cond_wait(cond, mutex)
{
lock(cond->__data.__lock);
unlock(mutex);
do {
unlock(cond->__data.__lock);
futex_wait(cond->__data.__futex);
lock(cond->__data.__lock);
} while(...)
unlock(cond->__data.__lock);
lock(mutex);
}
pthread_cond_broadcast(cond)
{
lock(cond->__data.__lock);
unlock(cond->__data.__lock);
futex_requeue(cond->data.__futex, cond->mutex);
}
Consider the simplified glibc calls::
/* caller must lock mutex */
pthread_cond_wait(cond, mutex)
{
lock(cond->__data.__lock);
unlock(mutex);
do {
unlock(cond->__data.__lock);
futex_wait(cond->__data.__futex);
lock(cond->__data.__lock);
} while(...)
unlock(cond->__data.__lock);
lock(mutex);
}
pthread_cond_broadcast(cond)
{
lock(cond->__data.__lock);
unlock(cond->__data.__lock);
futex_requeue(cond->data.__futex, cond->mutex);
}
Once pthread_cond_broadcast() requeues the tasks, the cond->mutex
has waiters. Note that pthread_cond_wait() attempts to lock the
......@@ -53,29 +54,29 @@ In order to support PI-aware pthread_condvar's, the kernel needs to
be able to requeue tasks to PI futexes. This support implies that
upon a successful futex_wait system call, the caller would return to
user space already holding the PI futex. The glibc implementation
would be modified as follows:
/* caller must lock mutex */
pthread_cond_wait_pi(cond, mutex)
{
lock(cond->__data.__lock);
unlock(mutex);
do {
unlock(cond->__data.__lock);
futex_wait_requeue_pi(cond->__data.__futex);
lock(cond->__data.__lock);
} while(...)
unlock(cond->__data.__lock);
/* the kernel acquired the mutex for us */
}
pthread_cond_broadcast_pi(cond)
{
lock(cond->__data.__lock);
unlock(cond->__data.__lock);
futex_requeue_pi(cond->data.__futex, cond->mutex);
}
would be modified as follows::
/* caller must lock mutex */
pthread_cond_wait_pi(cond, mutex)
{
lock(cond->__data.__lock);
unlock(mutex);
do {
unlock(cond->__data.__lock);
futex_wait_requeue_pi(cond->__data.__futex);
lock(cond->__data.__lock);
} while(...)
unlock(cond->__data.__lock);
/* the kernel acquired the mutex for us */
}
pthread_cond_broadcast_pi(cond)
{
lock(cond->__data.__lock);
unlock(cond->__data.__lock);
futex_requeue_pi(cond->data.__futex, cond->mutex);
}
The actual glibc implementation will likely test for PI and make the
necessary changes inside the existing calls rather than creating new
......
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